July 18, 2026 APM Plumbing & Electrical

Electrical Cable Identification — Colour Codes, Conductor Sizing, and Circuit Protection for UK Electricians

Cable identification is a fundamental skill for any UK electrician. Since the 2006 harmonisation with European standards, British electrical installations use a two-generation colour coding system — and existing buildings still contain old-colour wiring that must be correctly identified during alterations and extensions. This guide covers both systems, conductor sizing for common circuits, and the overcurrent protection required at each cable rating.

Why Cable Colour Coding Changed in 2006

Before 1 April 2006, UK cables followed BS 3036 colour conventions that differed from the rest of Europe. When the 17th Edition wiring regulations (BS 7671:2008) harmonised with CENELEC standards, the colour system changed. The transition period ran from 2004 to 2006, meaning installations begun before April 2006 could use the old colours throughout. Any additions or alterations to existing old-colour installations must be labelled at the distribution board and at every point where old and new cables meet.

Harmonised Colours: New System (Post-2006)

Twin and Earth (T&E) — Fixed Wiring

Core	New Colour (post-2006)
Line (Live)	Brown
Neutral	Blue
Earth	Green/Yellow (bare copper sleeved)

Old System (Pre-2006) — Still Found in Existing Buildings

Core	Old Colour (pre-2006)
Line (Live)	Red
Neutral	Black
Earth	Green/Yellow (bare copper sleeved)

Critical warning: In old-colour three-wire switch drops, the black core carries line voltage (switch wire). Always confirm conductor function with a voltage tester before working on existing wiring. Do not assume black = neutral in a switch drop.

Three-Phase Colour Coding

Three-phase installations use a three-core (or four-core with neutral) cable:

Core	New Colour	Old Colour
Line 1	Brown	Red
Line 2	Black	Yellow
Line 3	Grey	Blue
Neutral	Blue	Black
Earth	Green/Yellow	Green/Yellow

Note that new three-phase black and old three-phase neutral black create a direct identification conflict. Always verify with a phase rotation meter and voltage tester before making connections in mixed-generation three-phase installations.

Flexible Cable Colours

Flexible cables (for appliances, luminaires, and extension leads) use a slightly different convention:

Brown — Line
Blue — Neutral
Green/Yellow — Earth (protective conductor)

This matches the new fixed-wiring convention and has been the standard for flexibles in the UK since the 1970s, predating the fixed-wiring change.

Conductor Sizing for Common Domestic Circuits

The correct cable cross-section depends on the design current, installation method, grouping, and ambient temperature. The values below assume installation method B (enclosed in conduit or trunking, or in a wall, floor, or ceiling with thermally insulating material absent) at 30°C ambient with no grouping — the most common domestic scenario. Always verify against BS 7671 Appendix 4 for your specific installation conditions.

Ring Final Circuit — 32A MCB / 30mA RCD

Cable: 2.5mm² T&E
CPC (earth): 1.5mm²
Maximum floor area: 100m² (domestic), no limit on sockets
Spurs: permitted, but no more socket outlets per spur than on ring

Radial Socket Circuit — 20A MCB

Cable: 2.5mm² T&E
Maximum floor area: 50m²

Radial Socket Circuit — 32A MCB

Cable: 4mm² T&E
No floor area limit

Lighting Circuit — 6A MCB

Cable: 1.0mm² T&E
Maximum load: 1,380W (6A × 230V)
Typical: 8–12 downlights at up to 10W each — well within limit

Cooker Circuit — 32A or 40A MCB

Up to 10kW: 6mm² T&E, 32A MCB (using diversity per BS 7671 Table 4D)
Over 10kW: 10mm² T&E, 40A MCB
Always include a 45A cooker switch within 2m of the appliance

Shower Circuit — 40A or 45A MCB

Up to 9.5kW: 6mm² T&E, 40A MCB
10kW+: 10mm² T&E, 45A MCB
Must be RCD protected (30mA) per 18th Edition Regulation 701

Boiler/Heating Circuit — 6A MCB

Cable: 1.0mm² or 1.5mm² T&E
Fused connection unit (FCU) with 3A or 5A fuse at spur

Immersion Heater Circuit — 16A MCB

Cable: 2.5mm² T&E
Switched fused connection unit (SFCU) rated 20A at appliance

Earth Continuity and CPC Sizing

The circuit protective conductor (CPC) in T&E cable is bare copper, sized at half the main conductor cross-section for conductors up to 16mm²:

1.0mm² T&E → 1.0mm² CPC (same size for small cores)
1.5mm² T&E → 1.0mm² CPC
2.5mm² T&E → 1.5mm² CPC
4.0mm² T&E → 1.5mm² CPC (flat twin) or 2.5mm² (three-core)
6.0mm² T&E → 2.5mm² CPC
10.0mm² T&E → 4.0mm² CPC

CPCs must be sleeved green/yellow wherever accessible — bare copper in a junction box or accessory is non-compliant. Always carry green/yellow sleeving.

Cable Ratings — Installation Method Reference

BS 7671 defines current-carrying capacity by installation method (Table 4D2A for T&E). Key methods:

Method A: Enclosed in conduit in a thermally insulating wall — lowest rating
Method B: Enclosed in conduit on a wall, in trunking, or clipped to a surface — standard domestic reference method
Method C: Clipped direct — higher rating
Method 100/101: In free air or on cable tray — highest rating

When cables pass through thermal insulation (loft insulation, wall cavity), apply a derating factor — typically reducing the cable rating to that of Method A. A 2.5mm² T&E ring final circuit clipped direct might carry 27A, but if surrounded by 100mm of mineral wool insulation, the permitted current drops to as low as 16A, which would make a 32A MCB non-compliant.

Voltage Drop Limits

BS 7671 Table 4Ab specifies maximum voltage drop from the supply terminals to any fixed current-using equipment:

Lighting circuits: 3% (6.9V on 230V supply)
Other circuits: 5% (11.5V on 230V supply)

For long cable runs to outbuildings, garden circuits, or commercial premises, check voltage drop using the formula: V = (mV/A/m × I × L) / 1000, where mV/A/m figures come from BS 7671 Appendix 4. A 2.5mm² cable at 20A over a 30m single-way run may exceed the 5% limit, requiring 4mm² cable.

Circuit Identification Labels

BS 7671 Regulation 514.8 requires that every circuit be identifiable at the distribution board. Use durable labels — not handwritten tape — showing circuit function (e.g. "Kitchen Ring" or "Upstairs Lights"). Where old-colour wiring exists in the same installation, BS 7671 Regulation 514.14 requires a warning notice at the consumer unit:

"CAUTION — This installation has wiring colours to two versions of BS 7671. Great care should be taken before undertaking extension, alteration, or repair that all conductors are correctly identified."

Marking Conductors at Accessories

At any point where a conductor changes function — particularly in switch drops and intermediate switching — sleeving must be applied:

Blue core used as line in switch drop: brown sleeve at both ends
Grey or black core used as line in three-way switching: brown sleeve
Old red core: no change needed (it already indicates line)
Old black used as switch wire: brown or red sleeve to show it is not neutral

Conduit and Trunking Cable Capacity

When pulling cables into conduit or trunking, the space factor must not exceed 45% of internal cross-section (BS 7671 Appendix 5). Standard 20mm circular conduit has an internal area of approximately 201mm² — meaning the total cross-section of cables installed must not exceed 90mm². Overcrowding generates heat and reduces current-carrying capacity.

Testing After Installation

Cable identification must be verified electrically as well as visually. The four mandatory tests for any new circuit are:

Continuity of CPCs (R1 + R2 test)
Continuity of ring final conductors (ring circuit only)
Insulation resistance — minimum 1MΩ between live conductors and earth
Polarity — line connected to centre pin of E14/E27 lamps, line to L terminal of switches

See our guide to electrical installation testing for test methods, acceptable values, and certification requirements.

Working on Old Installations Safely

When extending or altering an existing installation with pre-2006 wiring:

Identify all conductors with a voltage tester before touching
Check for rubber-insulated or fabric-covered cables — these are likely 1960s or earlier and may have degraded insulation requiring full rewire
Confirm the incoming supply system (TN-S, TN-C-S, or TT) — this affects earthing arrangements and RCD requirements
Label the distribution board with the mixed-colour warning
Issue a Minor Works Certificate or Electrical Installation Certificate as appropriate

For further background on earthing systems and supply configurations, see our guide to electrical earthing systems. For RCD and RCBO circuit protection, refer to our article on RCD, RCCB, and RCBO protection. For consumer unit selection, see our consumer unit and MCB guide. For junction box wiring, refer to our junction boxes and enclosures article.